Electric vehicles, including battery, hybrid, and fuel cell electric vehicles, typically use an inverter in the form of a switch-mode power supply to provide three phase operating power to the vehicle's electric drive motor. The inverter design most commonly used is a pulse width modulated (PWM) voltage source inverter which utilizes power transistors that can supply the high currents needed to satisfy the torque demands required by the vehicle drive motor. The inverter switches power to the motor windings from a high voltage bus of approximately 350-400 Vdc.
In addition to the drive motor used to power the vehicle wheels, an electric vehicle normally includes various auxiliary drive motors to operate a variety of different vehicle systems. Some examples of these auxiliary drive motors include liquid coolant pumps, traction control motors, climate control compressor motors, electronic power steering pumps, as well as other blowers and fans. Although suitable DC motors are commercially available that can operate directly off the 350-400 volt bus, this high voltage operation is generally considered undesirable for various reasons including, for example, reduced motor life due to arcing and other commutation problems. Accordingly, lower voltage motors are sometimes used that can be driven from an intermediate voltage power bus operating at, for example, a fixed 42-48 Vdc. This intermediate voltage supply can be produced using a DC--DC converter that draws operating power from the high voltage bus and develops the fixed DC voltage using conventional DC--DC conversion techniques.
In addition to the intermediate voltage supply, some of the auxiliary power systems utilized on an electric vehicle require a standard vehicle voltage supply of 12 Vdc. This lower voltage bus is typically produced by a separate DC--DC converter that also draws its operating power from the high voltage bus. While providing a more desirable low voltage operation of the various auxiliary vehicle power systems, these 12 and 42 volt dedicated DC--DC converters bring with them a number of inherent disadvantages as well. In particular, each of the DC--DC converters used typically include their own input filter, control circuitry, and power transistors, the last of which requires its own thermal management--usually by way of connecting the converter into the liquid coolant system used for cooling the inverter power transistors. As a result, the DC--DC converters not only take up substantial room in the vehicle power electronics chassis, but also add significantly to the overall vehicle power system cost.
Apart from the use of DC--DC converters operating off the high voltage bus, rotary converters operating off the drive motor are sometimes used to generate the intermediate and lower voltage supplies needed by the vehicle. However, these converters are generally bulky, heavy, and inefficient. Moreover, for rotary converters in the form of an "alternator" that operates off the vehicle heat engine, the converter will not work when the engine is stopped. Where the rotary converter is run off the shaft of an electric motor, a custom design is generally required since most automotive alternators cannot run at the high speeds (8,000-15,000 rpm) of a drive motor.
It is therefore a general object of this invention to provide an electric vehicle power system that eliminates the need for dedicated DC--DC converters to generate the intermediate and/or low voltage supplies utilized for the vehicle's auxiliary power systems.